Fuel cells are used as an electrical power source in many applications. In particular, fuel cells are proposed for use in automobiles to replace internal combustion engines. A commonly used fuel cell design uses a solid polymer electrolyte (“SPE”) membrane or proton exchange membrane (“PEM”) to provide ion transport between the anode and cathode.
In proton exchange membrane type fuel cells, hydrogen is supplied to the anode as fuel and oxygen is supplied to the cathode as the oxidant. The oxygen can either be in pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells typically have a membrane electrode assembly (“MEA”) in which a solid polymer membrane has an anode catalyst on one face, and a cathode catalyst on the opposite face. The anode and cathode layers of a typical PEM fuel cell are formed of porous conductive materials, such as woven graphite, graphitized sheets, or carbon paper to enable the fuel and oxidant to disperse over the surface of the membrane facing the fuel- and oxidant-supply electrodes, respectively. Each electrode has finely divided catalyst particles (for example, platinum particles) supported on carbon particles to promote oxidation of hydrogen at the anode and reduction of oxygen at the cathode. Protons flow from the anode through the ionically conductive polymer membrane to the cathode where they combine with oxygen to form water which is discharged from the cell. The MEA is sandwiched between a pair of porous gas diffusion layers (“GDL”) which, in turn, are sandwiched between a pair of non-porous, electrically conductive elements or plates. The plates function as current collectors for the anode and the cathode, and contain appropriate channels and openings formed therein for distributing the fuel cell's gaseous reactants over the surface of respective anode and cathode catalysts. In order to produce electricity efficiently, the polymer electrolyte membrane of a PEM fuel cell must be thin, chemically stable, proton transmissive, non-electrically conductive and gas impermeable. In typical applications, fuel cells are provided in arrays of many individual fuel cell stacks in order to provide high levels of electrical power.
Reducing the Pt loading and improving the ORR activity of the cathode catalyst has become one of the most difficult challenges on the road of commercializing the PEM fuel cell vehicle. Graphite particle supported core-shell electrocatalysts containing a continuous thin layer of Pt or Pt alloy shell overlaid on non-noble metal substrate particles can potentially overcome this critical challenge. A core-shell electrocatalyst concept where Pt is dispersed only on the surface of the nanoparticles is very promising due to its high activity and high dispersion of Pt. However, in a high temperature and highly acidic condition like those in a PEM fuel cell, choices of the core are very limited and often costly. Most less-costly candidates are either unstable or have poor adhesion to Pt resulting in poor Pt deposition. Although refractory metal alloys can potentially be good candidates, making these metal alloys into a nanoparticulate form is very challenging.
Pt ML/shell electrocatalysts' catalytic activities can be tuned by appropriate core/substrate. Pd and its alloys are still the best candidates as a substrate to support Pt ML/shell. Tungsten-M (M=Pd, Ni, Co, etc.) alloys have not been tested as stable core materials for Pt ML/shells. It has been reported in a theoretical study that the Pd dissolution potential in Pd/W system is higher than that of Pd(111), and even higher than the Pt dissolution potential. A strong binding energy between M and tungsten (W) should prevent M from being pulled up on to the top Pt ML/shell during the potential cycling and should lead to a higher oxygen reduction reaction (ORR) activity.
Accordingly, there is a need for an improved methodology for making core-shell electrocatalysts for fuel cell applications.